Digital burette and method for displaying the dose volume in said digital burette

ABSTRACT

The invention relates to a digital burette having a manual drive for suctioning and applying an adjustable dose volume, a gear unit connected downstream and a digital display device controlled thereby for setting the dose volume. The control comprises an incremental encoder which is functionally connected to the gear unit, at least one sensor for detecting the signals produced by the incremental encoder and a processor connected to the sensor and the display device for calculating the dose volume corresponding to the number of signals. The incremental encoder is, in particular, a sector disc with two groups of sectors of different magnetic field strength disposed alternately in the peripheral direction of the sector disc. A sensor detects the number of turns of the sector disc. To save energy, the power supply of the burette is ensured by a solar cell and two spaced sensors are provided for detecting the number of turns of the sector disc to reduce the clock frequency of the processor for a same measuring accuracy. The invention also relates to a method for displaying the dose volume of such a digital burette.

[0001] The invention concerns a digital burette having a manual drivefor suctioning and delivering an adjustable dose volume, a downstreamgear and a digital display device controlled thereby for setting thedose volume, wherein the control comprises an incremental encoder whichis operatively connected to the gear, at least one sensor for detectingthe signals produced by the incremental encoder and a processor,connected to the sensor, for calculating the dose volume correspondingto the number of signals, the processor being connected to the digitaldisplay device. The invention also concerns a method for displaying thedose volume in said digital burette.

[0002] Digital burettes are used for a plurality of volumetricaldeterminations, such as titration, for exact dosing of defined liquidvolumes. Of particular interest is the possibility to pre-set andreproduce the dose volume to be delivered which a digital displaydevice, e.g. a display, indicates.

[0003] The operation of such digital burettes is based on a liftingpiston guided in a pipetting channel for suctioning the liquid to bedosed, wherein the lifting piston usually communicates with the liquidvia an air cushion. The lifting piston is manually driven, e.g. actuatedby a turning handle, wherein the rotation is transmitted via amechanical gear to the lifting piston for suctioning or discharging thedesired liquid volume. An incremental encoder is operatively connectedto the gear and a sensor detects the signals generated by theincremental encoder to detect the dose volume corresponding to thenumber of turns of the turning handle. The sensor is connected to aprocessor which calculates the dose volume from the signals, displayssame on a digital display device e.g. a liquid crystal display, lightdiodes or the like.

[0004] The incremental encoder may be a slotted disc with slotsprovided, one behind the other, in the peripheral direction. An opticalsensor, e.g. a light barrier, detects the number of turns of the slotteddisc by detecting the number of slots passing the sensor. To recognizethe direction of rotation of the slotted disc and therefore the liftingdirection of the lifting piston (suction or pressure stroke), a secondoptical sensor, e.g. a second light barrier or a forked light barriermay be provided with which the direction of rotation of the slotted discis determined from the order of the signals produced by the two sensors.

[0005] DE 38 18 531 A1 discloses a piston burette having an incrementalencoder in the form of a slotted disc cooperating with a forked lightbarrier to determine the rotational direction of the slotted disc andthe stroke direction of the piston. EP 0 559 223 A1 discloses a bottlefilling device having a similar control.

[0006] The incremental encoders may also be sector discs with two groupsof sectors of different magnetic field strength, wherein the sectors ofthe two groups are alternately disposed in the peripheral direction ofthe sector disc. The sectors may be formed e.g. by permanent magnetswhich induce alternately different and/or opposite magnetic fields.Alternatively, only every second sector of the sector disc may comprisea magnet with the sectors disposed between the magnets beingnon-magnetic. A sensor is provided which is sensitive to the magneticfield and functions like a magnetic switch to open or close exclusivelyin response to the magnetic field induced by the respective magnet fordetecting the number of turns of the sector disc. The currentconsumption of these sensors is advantageously small since they requireonly a voltage which is sufficient to detect the opening or closingposition while the opening and closing process per se is effected purelymechanically due to the force field induced by the magnet of the sectordisc. The sensor produces a substantially rectangular voltage or currentsignal when the sector disc is turned.

[0007] Digital burettes consume a relatively large amount of currentsince the digital display device and the overall control includingsensors and, in particular, processor require current. The processorrequires the greatest amount of current in dependence on its clockfrequency. Replaceable dry batteries or rechargeable accumulators areused as power supplies. The non-productive times during replacement orrecharging thereof are disadvantageous. They also fail prematurely underthe often-corrosive conditions in the laboratory and can damage theburette, in particular when electrolyte is discharged. Digital buretteswith lithium cells are also known which are soldered onto a boardtogether with the electric and/or electronic components. Lithium cellsof this type are relatively robust but the regular replacement of thecells requires dismantling or replacement of the entire board and istherefore demanding and expensive. Down times are created sincelong-term storage of the boards would impair the lithium cells.

[0008] It is the underlying object of the invention to overcome thesedisadvantages.

[0009] This object is achieved a digital burette of the above mentionedkind characterized in that at least one solar cell provides power to thecontrol and a sector disc is provided as incremental encoder having twogroups of sectors of different magnetic field strength which arealternately disposed in the peripheral direction of the sector disc, andthe number of turns of the sector disc is detected using at least onesensor which is sensitive to the magnetic field, preferably a Reedsensor. The solar cell also serves as power supply for the digitaldisplay device.

[0010] The inventive use of a solar cell as a power supply for thedigital burette avoids the need for replacement of batteries,accumulators or the like. The solar cells always provided safe andreliable operation for the digital burette. They are advantageouslydisposed on a side of the digital burette facing away from the deliverymember e.g. on its rear or upper side.

[0011] Since the available surface on the casing of the digital buretteis small and the arrangement of the solar cells is locally confined, thesurface of the solar cells must be minimized. This also requires theenergy consumption of the digital burette to be kept low. This is alsoaddressed by the configuration of the digital burette in that theincremental encoder is a sector disc with two groups of sectors ofdifferent magnetic field strength which are alternately disposed in theperipheral direction of the sector disc, and at least one magneticfield-dependent sensor, preferably a Reed sensor, for detecting thenumber of turns of the sector wheel. As previously noted, the currentconsumption of these sensors is advantageously small since they requireonly a voltage which is sufficient to detect the opening or closingposition while the opening and closing process per se is effected purelymechanically due to the force field induced by the magnet of the sectordisc.

[0012] In an preferred embodiment, two sensors are disposed at aseparation in the peripheral direction of the sector disc which aresensitive to the magnetic field, preferably Reed sensors, forcalculating the dose volume in dependence on the number of turns of thesector disc, wherein the angular separation of the sensors differs froman integer multiple of the angle between the sectors of the sector disc.This design permits considerable reduction in the clock frequency of theprocessor for a given measurement accuracy, e.g. by approximately half,which permits considerable savings in power such that when batteries oraccumulators are provided as the energy source, the replacementintervals are considerably increased or—in the preferred case when solarcells are used—the surface of the solar cells required for the necessaryamount of current, is considerably reduced and the solar cells can bedirectly disposed on a surface section of the housing of the digitalburette.

[0013] As already mentioned, digital burettes are known which utilizedifferent sensor technology, i.e. a slotted disc with two light barriers(DE 38 18 531 A1, EP 0 559 223 A1). The second light barrier, however,serves exclusively for detecting the turning direction of the slotteddisc or the lifting direction of the lifting piston and a reduction inthe clock frequency of the processor necessarily produces anunacceptable reduction in the measuring accuracy.

[0014] In accordance with the invention, both sensors serve to calculatethe dose volume in dependence on the number of turns of the sector discby supplying the signals of the switching-on and off processes of bothsensors to the processor for calculating the dose volume. In thisfashion, a total of two switching-on and off processes take place whenthe sector disc is turned by an angle which corresponds to the angle ofone sector, i.e. the number of signals per turn of the sector disc isincreased resulting in an increase in the measuring accuracy. Theoverall signal also indicates the position of the sensors with respectto the sectors of the sector disc and when the lifting piston of thedigital burette is moved quickly as the sector disc is rotated rapidly,reliable detection of the number of turns of the sector disc is possiblewhen the processor merely registers whether both sensors are in theopened and/or closed position, a situation which occurs only once whenthe sector disc is rotated past two sectors. The corresponding dosevolume can also be determined if the processor only evaluates one of thetwo signals produced by the sensors when the sector disc is movedquickly, i.e. merely the switching-on and/or off processes of onesensor. When the sector disc is turned slowly or has stopped, the exactposition of the sector disc which corresponds to the dose volume, can bedetermined by the relative position of the sector disc with respect tothe two sensors or by means of the two signals produced by the sensors,e.g. the switching-on and off processes of both sensors. In thisfashion, the clock frequency of the processor and the current requiredfor operation can be reduced for rapid motion of the lifting pistonwithout sacrificing measuring accuracy. Naturally, the sequence of thesignals of the two sensors or the overall signal also indicates theturning direction of the sector disc and therewith the lifting directionof the lifting piston as is known per se for digital burettes having twooptical sensors.

[0015] Although the sensors of such a control may, in principle,comprise any suitable sensors, Reed sensors-are of primary interest dueto their low current consumption and since they require no current forthe switching process per se and only low voltages for determining theopening and closing position, wherein e.g. the overall voltage or theoverall current of both sensors is transferred to the processor.

[0016] A further development of the invention provides that the powersupply of at least some electrical and/or electronic components of thecontrol is controlled in dependence on the action of the gear.

[0017] The energy supply to the sensors may be switched off when thegear has stopped or be switched on again when the gear is in action e.g.by means of a mechanical pulse. The power consumption of the processorcan also be reduced by lowering its clock frequency when the gear hasstopped and by increasing it again when the gear is in motion. This mayalso be effected by means of a mechanical pulse or directly upon receiptof the signal of at least one of the sensors.

[0018] The power supply of the digital display device can be switchedoff with a preset delay when the gear has stopped, wherein the displaydisappears e.g. after a certain time.

[0019] The processor preferably has a minimum operating voltage of atmost 2 V. These known processors function safely up to a minimumoperating voltage of 2 V and currently have the smallest powerconsumption, although the electronic industry continues to furtherminiaturize the processors and further reduce the minimum operatingvoltage. It is of course advisable to provide the inventive digitalburette with these commercially available processors of minimumoperating voltage to keep the power consumption as small as possible. Asa further development, substitution of the currently availableprocessors with new more energy-saving processors is envisioned.

[0020] In a preferred embodiment, the display device has an associatedcurrent-less storage for recording at least the last selected dosevolume. In this manner, the last selected dose volume is visible evenafter relatively long stoppage of the digital burette, which isparticularly desirable when the digital burette is used for a series ofexperiments.

[0021] The invention also concerns a method for displaying the dosevolume of a digital burette having a manual drive for suctioning anddischarging an adjustable dose volume, a downstream gear and a digitaldisplay device controlled thereby for setting the dose volume, whereinthe control comprises an incremental encoder operatively connected tothe gear in the form of a sector disc with two groups of sectors ofdifferent magnetic field strength disposed alternately in the peripheraldirection of the sector disc, at least one sensor, preferably a Reedsensor, for detecting the number of turns of the sector disc and aprocessor connected to the sensor for calculating the dose volumecorresponding to the number of turns of the sector disc, the processorbeing connected to the digital display device. The inventive method ischaracterized in that the signals are transmitted to the processor bytwo sensors disposed in the peripheral direction of the sector disc at aseparation which is not an integer multiple of the angle of the sectorsof the sector disc and the dose volume is calculated by means of the twosignals at least when the gear is moved slowly. The two signals alsoadvantageously detect the direction of rotation of the sector wheel.

[0022] The dose volume is advantageously calculated, at least duringslow motion of the gear, by means of the number of switching-on and offprocesses of both sensors, e.g. from the sum of the current or voltagepulses emitted by the sensors to the processor during switching on andoff.

[0023] In a further development, the processor is programmed with apresettable value of the number of turns of the sector disc such thatwhen the number of turns of the sector disc is smaller than thepredetermined value, the dose volume is calculated on the basis of thenumber of switching on and off processes of both sensors. When thenumber of turns of the sector disc is larger than the predeterminedvalue, the dose volume is calculated exclusively on the basis of theswitching-on and off processes of one of the two sensors. If the numberof turns of the sector disc is larger than the predetermined value, thedose volume is calculated exclusively on the basis of the switching-onprocesses or exclusively on the basis of the switching-off processes ofone of the two sensors. In this fashion, only one switching process isutilized when the sector disc is rotated through two sectors, wherein arelatively low and current-saving processor clock frequency issufficient to detect these states. When the sector disc is turned slowlyor rests, a small dose volume can also be determined on the basis of thenumber of switching-on and off processes of both sensors or from the sumof the current or voltage pulses provided to the processor by the twosensors during switching on and off, wherein the maximum error issmaller than the stroke length of the lifting piston of the digitalburette associated with a rotation of the sector disc by half a sector.

[0024] It may be advantageous to program the processor with twodifferent presettable values of the number of turns of the sector disc.In this case, the dose volume is calculated on the basis of the numberof the switching-on and off processes of both sensors when the number ofturns of the sector disc is smaller than the predetermined low value.When the number of turns of the sector disc is between the twopredetermined values, the dose volume is determined on the basis of thenumber of switching-on and off processes of one of the two sensors. Whenthe number of turns of the sector disc is larger than the predeterminedhigher value, the dose volume is determined exclusively on the basis ofthe switching-on or off processes of one of the two sensors. This allowsan even finer graduation of the operating states and clocking of theprocessor.

[0025] As indicated above, the current supply of at least someelectrical and/or electronic components of the control is controlled independence on the action of the gear. In particular, power supply to thesensors can be switched off when the gear is at rest. The clockfrequency of the processor is preferably reduced when the gear hasstopped.

[0026] A further preferred energy reduction consists in that the powersupply of the digital display device is switched off with a preset delaywhen the gear has stopped.

[0027] The processor is advantageously loaded with a minimum operatingvoltage of at most 2V. The voltage applied to the processor duringoperation can be selected e.g. between approximately 2 and 3.5V.

[0028] In a preferred embodiment, at least the dose volume displayedlast by the display device is stored without current to provide accessto the last selected dose volume even after longer stoppages of thedigital burette.

[0029] Although batteries or accumulators may in principle be providedas power supply to the electrical and/or electronic components of thedigital burette, wherein this operating time is considerably increasedby the inventive method, the control and digital display device isadvantageously supplied with current from at least one solar cell.

[0030] The invention is explained in more detail below with reference toone embodiment and the drawings.

[0031]FIG. 1 shows a perspective view of one embodiment of the driveblock of a digital burette;

[0032]FIG. 2 shows a schematic view of the incremental encoder of thedigital burette in accordance with FIG. 1 with two sensors;

[0033]FIG. 3 shows the signal dependence on one sensor in accordancewith FIG. 2;

[0034]FIG. 4 shows the signal dependence on the other sensor inaccordance with FIG. 2;

[0035]FIG. 5 shows the signals of the sensors processed by the processorunder rapid movement of the gear to calculate large dose volumes; and

[0036]FIG. 6 shows signals of the sensors processed by the processorwhen the gear is moved slowly, used to calculate small dose volumes.

[0037]FIG. 1 shows a drive block 10 disposed in the head of a digitalburette. The drive block 10 has a manual drive 11 in the form of arotating handle 12. An identical rotating handle is optionally locatedon the opposite side, disposed on a common axle. The axle is connectedto a piston rod 16 of a lifting piston 17 via a mechanical gear 15formed from toothed wheels 13 and pinions 14. The piston rod 16 hasteeth 18 in its upper section facing the output side of the gear 15. Thelifting piston 17 is guided in a pipetting channel 19 and sealed withrespect thereto by means of a sealing lip 20 to suction or discharge thedesired liquid volume.

[0038] To detect the dose volume which corresponds to the number ofturns of the turning handle 12, a control is provided with anincremental encoder 21, which is operatively connected to the gear 15,in the form of a sector disc 1 (FIG. 2) having two groups of sectors 2,3 of different magnetic field strength which are alternately disposed inthe peripheral direction u of the sector disc 1. The sectors 2, 3 areprovided e.g. with permanent magnets of different polarity (northpole/south pole) such that the magnets of two neighboring sectors 2, 3of the sector disc 1 each induce a magnetic field having opposite fieldlines. Alternatively, the sectors 2 of the sector disc 1 can havemagnets of the same polarity while the sectors 3 are not magnetic. Inthe embodiment shown, the sector disc 1 has a total of eight sectors 2,3.

[0039] Two Reed sensors S1, S2 are provided in the peripheral region ofthe sector disc 1 and are disposed one behind the other in theperipheral direction thereof at an angular separation α which is e.g.approximately ⅔ of the angle β between the sectors 2, 3 of the sectordisc 1. The sensors S1, S2 are connected to a processor (not shown)which calculates the dose volume from the signals generated by thesensors S1, S2 when the sensor disc 1 is in action, which is thendisplayed by a digital display device 23 (FIG. 1) e.g. a liquid crystaldisplay. The processor and the display device 23 are supplied withcurrent by a solar cell 22 (FIG. 1). The solar cell 22 applies a smalloperating voltage to the sensors S1, S2 and each sensor S1, S2 producesa substantially rectangular signal (FIGS. 3, 4) when the sector disc 1is turned. The rectangular signal is thereby produced by opening andclosing the Reed sensors S1, S2 which act like a magnetic switch,wherein the sectors S1, S2 are opened e.g. by the magnetic field of thesectors 2 and are closed by a magnetic field of the sectors 3 which isdifferent with respect thereto. As shown in FIGS. 3 and 4, the angle βof the sectors 2, 3 corresponds to the width of a signal and the angleseparation a between the two sensors S1, S2, is represented by the phaseshift between the two signals T1, T2, from which the peripheraldirection of the sector disc 1 can be determined.

[0040] In the present embodiment (see FIGS. 5 and 6), the controldifferentiates between large and small dose volume or between fast andslow action of the turning handle. The processor can be programmed witha preset value of the number of turns of the sector disc 1 and an inputof the processor monitors the actual rotating speed of the sector disc 1and compares it with the predetermined value.

[0041]FIG. 5 shows the signals I_(S) of the sensors S1, S2 evaluated bythe processor during fast action of the turning handle 12 forcalculating the dose volume displayed by the display device 23. FIG. 5shows that during fast action in the present embodiment, only theswitching-on processes, i.e. only the respective rising edge of thesignal from sensor S1 (FIG. 3) is utilized. This switching processoccurs once after each rotation of the sector disc 1 through two sectors2, 3. Consequently, when the sector disc 1 is rotated faster than thepredetermined value, only every fourth switching process of both sensorsS1, S2 is used to calculate the dose volume. If one increment is e.g. 1μl, each of the signals I_(S) shown in FIG. 5 corresponds to 4 μl. Theclock frequency and therefore the current consumption of the processorcan be reduced in this fashion when the gear is in fast action.

[0042] During fast action of the gear 15 (large dose volume), arelatively coarse resolution (registration of the switching-on processesof only one of the two sensors S1, S2) is sufficient, but for slowaction of the gear 15 (small dose volume) the resolution should berelatively high to keep dosing errors small. For this reason, when thenumber of turns of the sector disc 1 is smaller than the predeterminedvalue, the dose volume is calculated using the switching on and offprocesses of both sensors S1, S2 (see FIG. 6). For slow action of thegear 15, the switching-on and off processes of both sensors S1, S2, i.e.the rising and falling edges of the signals of the sensors S1 and S2(FIGS. 3 and 4) are registered, i.e. a total of four signals isevaluated. If an increment corresponds e.g. to 4 μl, each of the signalsI_(S) reproduced in FIG. 6 corresponds to 1 μl or four signals I_(S)correspond to 4 μl each. The two sensors S1, S2 produce increasedmeasuring accuracy when the gear 15 is in slow action by producing foursignals I_(S) per rotation of the sector disc 1 through the angle β ofone sector 2, 3. In this fashion, the dose volume can be preciselydetermined from the overall signal I_(L) even for abruptly terminatedfast rotation of the sector disc 1.

[0043] The power savings resulting from the reduced current consumptionof the processor due to reduction in its clock frequency in dependenceon the action of the gear 15 is approximately 30% compared to the powerconsumption of a digital burette having only one sensor.

1. Digital burette with a manual drive (11) for suctioning anddelivering an adjustable dose volume, a downstream gear (15) and adigital display device (23) controlled thereby for setting the dosevolume, wherein the control comprises an incremental encoder (21) whichis operatively connected to the gear (15), at least one sensor (S1, S2)for detecting the signals produced by the incremental encoder (21) and aprocessor, connected to the sensor, for calculating the dose volumecorresponding to the number of signals, the processor being connected tothe digital display device (23), characterized in that at least onesolar cell (22) provides power to the control and a sector disc (1) isprovided as incremental encoder (21) having two groups of sectors (2, 3)of different magnetic field strength which are alternately disposed inthe peripheral direction (u) of the sector disc (1), and the number ofturns of the sector disc (1) is detected using at least one sensor (S1,S2) which is sensitive to the magnetic field, preferably a Reed sensor.2. Digital burette according to claim 1, characterized in that the solarcell (22) also serves for supplying power to the digital display device(23).
 3. Digital burette according to claim 1 or 2, characterized by twosensors (S1, S2) which are sensitive to the magnetic field, preferablyReed sensors, and which are disposed at a mutual separation in theperipheral direction (u) of the sector disc (1) for calculating the dosevolume in dependence on the number of turns of the sector disc (1),wherein the angular separation (α) of the sensors (S1, S2) differs froman integer multiple of the angle (β) between the sectors (2, 3) of thesector disc (1).
 4. Digital burette according to any one of the claims 1through 3, characterized in that the power supply of at least someelectrical and/or electronic components of the control are controlled independence on the action of the gear (15).
 5. Digital burette accordingto claim 4, characterized in that the power supply of the sensors (S1,S2) can be switched off when the gear (15) is at rest.
 6. Digitalburette according to claim 4, characterized in that the power supply ofthe processor can be reduced by reducing its clock frequency when thegear (15) is at rest.
 7. Digital burette according to any one of theclaims 1 through 6, characterized in that the power supply of thedigital display device (23) can be switched off with a preset delay whenthe gear (15) is at rest.
 8. Digital burette according to any one of theclaims 1 through 7, characterized in that the processor has a minimumoperating voltage of at most 2V.
 9. Digital burette according to any oneof the claims 1 through 8, characterized in that the display device (23)has an associated current-less storage for at least the last set dosevolume.
 10. Method for displaying the dose volume of a digital burettehaving a manual drive (11) for suctioning and delivering an adjustabledose volume, a downstream gear (15) and a digital display device (23)controlled thereby for adjusting the dose volume, wherein the controlcomprises an incremental encoder (21), which is operatively connected tothe gear (15), in the form of a sector disc (1) having two groups ofsectors (2, 3) with different magnetic field strength which arealternately disposed in the peripheral direction (u) of the sector disc(1), and with at least one sensor (S1, S2) which depends on the magneticfield for detecting the number of turns of the sector disc (1) and aprocessor, connected to the sensor (S1, S2) for calculating the dosevolume in correspondence with the number of turns of the sector disc(1), the processor being connected to the digital display device (23),characterized in that the control is supplied with power by at least onesolar cell (22) and the signals are provided for the processor by twosensors (S1, S2) disposed in the peripheral direction (u) of the sectordisc (1) at a separation which is not an integer multiple of the angle(β) between the sectors (2, 3) of the sector disc (1), the dose volumebeing calculated on the basis of these two signals, at least during slowaction of the gear (15).
 11. Method according to claim 10, characterizedin that the dose volume is calculated at least during slow action of thegear (15) by the number of the switching-on and off processes of bothsensors (S1, S2).
 12. Method according to claim 10 or 11, characterizedin that the processor is programmed with a pre-settable value of thenumber of turns of the sector disc (1) and when the number of turns ofthe sector disc (1) is smaller than the predetermined value, the dosevolume is calculated on the basis of the number of switching-on and offprocesses of both sensors (S1, S2), while for a number of turns of thesector disc (1) which is larger than the predetermined value, the dosevolume is calculated only on the basis of the switching-on and offprocesses of one of the two sensors (S1, S2).
 13. Method according toclaim 12, characterized in that when the number of turns of the sectordisc (1) is larger than the predetermined value, the dose volume iscalculated only on the basis of the switching-on processes of one of thetwo sensors (S1, S2).
 14. Method according to any one of the claims 10through 13, characterized in that the power supply to at least someelectrical and/or electronic components of the control is controlled independence on the action of the gear (15).
 15. Method according to claim14, characterized in that the power supply of the sensors (S1, S2) isswitched off when the gear (15) has stopped.
 16. Method according toclaim 14, characterized in that the clock frequency of the processor isreduced when the gear (15) has stopped.
 17. Method according to any oneof the claims 10 through 16, characterized in that the power supply tothe digital display device (23) is switched off with predetermined delaywhen the gear (15) has stopped.
 18. Method according to any one of theclaims 10 through 17, characterized in that the processor is loaded witha minimum operating voltage of at most 2V.
 19. Method according to anyone of the claims 10 through 18, characterized in that at least the dosevolume which was displayed last by the display device (23) is storedwithout using current.
 20. Method according to any one of the claims 10through 19, characterized in that the digital display device (23) isalso supplied with power from at least one solar cell (22).